The aforementioned application Ser. No. 09/789,313 and Provisional Application No. 60/183,613 are hereby incorporated by reference in their entirety.
The present invention relates to an electron beam accelerator, and more particularly to a system for dynamically controlling a cathode current flowing in the accelerator to reduce overshoot in the output voltage of the step-up transformer employed by the accelerator.
Particle acceleration technology has been known and used for a variety of applications for many years. Much of the technology was developed in the 1950""s and 1960""s for scientific research in the study of matter and its subatomic composition. In subsequent years, industrial applications of particle accelerators, particularly electron beam accelerators, have been identified. Such applications include curing of resins used in the manufacture of composite materials, cross-linking polymers and irradiation of food to eliminate harmful parasites and pathogens.
The energy of a moving electron is given in units of electron volts (eV) which correspond to the velocity that an electron would achieve if it were attracted to a positive static voltage V. The typical electron energies for food irradiation purposes range from 1 to 10 million electron volts (MeV). Higher energy electrons are able to penetrate to greater depths, but typically require more complex and costly equipment to generate. Penetration to greater depths has the advantage of allowing irradiation processing of thicker materials, but has the disadvantage of requiring greater shielding to reduce the radiation exposure of operating personnel to safe levels.
The typical technology used to accelerate electrons to the 1 to 10 MeV energy range involves the use of a very high power microwave pulse driving a precisely tuned microwave waveguide. The construction of the waveguide and the generation of the very high power microwave pulse are complex and involve processes that are consequently rather costly. For relatively low electron energies of up to several hundred KeV, a static direct current voltage source is typically used. A very common application of this method is x-ray generation which is commonly used for medical and industrial imaging. However, energies of 1 to 10 MeV would require the generation of a static voltage of 1 to 10 megavolts (MV). Such high voltages are quite difficult to manage without dielectric breakdown and resultant failure. A system that provides a sufficiently high voltage to achieve electron energies of greater than about 1 MeV while reducing or eliminating the risk of dielectric breakdown would be an improvement to the state of the art.
In addition to this, typical electron beam accelerators are constrained to operate at predetermined fixed energy levels. Certain irradiation applications would benefit from a capability to generate electrons at variable energy levels depending on certain physical characteristics of the material to be processed. For example, de-infesting of grain may require relatively shallow electron beam penetration to kill parasites or pathogens while preserving the germination potential of the seed. Different types of seeds may need different electron beam energy to effectively de-infest the seeds. A variable energy accelerator would make it possible to process these or other materials that need variable penetration and exposure.
The present invention is a direct injection electron beam accelerator system that includes a direct current voltage source and a pulse forming network coupled through a resistor to the direct current voltage source. A high power switching device is coupled between the direct current voltage source and the pulse forming network. A pulse control circuit is connected to control the high power switching device to selectively allow a current to flow to the pulse forming network. A step-up transformer is coupled to the pulse forming network, and a cathode structure is coupled to the high voltage output of the step-up transformer. An anode structure is spaced from the cathode structure, and has a first voltage associated therewith such that a voltage difference exists between the cathode structure and the anode structure. This voltage difference creates an electron beam flowing between the cathode structure and the anode structure. An electron beam output is adjacent to the anode structure. A control grid is located between the cathode structure and the anode structure. A control grid drive circuit is operatively coupled to the pulse control circuit and the control grid, and is operable to apply a time-varying second voltage to the control grid synchronized with the pulse control circuit. The control grid therefore effectively provides a dynamic load on the high voltage output of the step-up transformer that prevents overshoot in the transformer output, reducing the risk of dielectric breakdown and failure due to transient high voltages.